Embodiments of the present invention relate to control of managed pressure drilling systems, e.g. control of the hydraulics involved in managed pressure drilling, which system may be used in drilling a subterranean borehole, and particularly, but not exclusively, for the purpose of extracting hydrocarbons from a subterranean reservoir. The systems may use a single phase drilling fluid or may use multiple phases. The systems may be underbalanced. The systems may include produced formation fluids.
The drilling of a borehole is typically carried out using a steel pipe known as a drillstring with a drill bit coupled on the lower most end of the drillstring. The entire drillstring may be rotated using an over-ground drilling motor, or the drill bit may be rotated independently of the drillstring using a fluid powered motor or motors mounted in the drillstring just above the drill bit. As drilling progresses, a flow of drilling fluid is used to carry the debris created by the drilling process out of the borehole. The drilling fluid is pumped through an inlet line down the drillstring to pass through the drill bit, and returns to the surface via an annular space between the outer diameter of the drillstring and the borehole (generally referred to as the annulus or the drilling annulus).
Drilling fluid is a broad drilling term that may cover various different types of drilling fluids. The term “drilling fluid” may be used to describe any fluid or fluid mixture used during drilling and may cover such things as air, nitrogen, misted fluids in air or nitrogen, foamed fluids with air or nitrogen, aerated or nitrified fluids to heavily weighted mixtures of oil or water with solid particles.
The drilling fluid flow through the drillstring may be used to cool the drill bit. In conventional overbalanced drilling, the density of the drilling fluid is selected so that it produces a pressure at the bottom of the borehole (the “bottom hole pressure” or “BHP”), which is high enough to counter-balance the pressure of fluids in the formation (the “formation pore pressure”). By counter-balancing the pore pressure, the BHP acts to prevent the inflow of fluids from the formations surrounding the borehole. However, if the BHP falls below the formation pore pressure, formation fluids, such as gas, oil and/or water may enter the borehole and produce what is known in drilling as a kick. By contrast, if the BHP is very high, the BHP may be higher than the fracture strength of the formation surrounding the borehole resulting in fracturing of the formation. When the formation is fractured, the drilling fluid—which is circulated down the drillstring and through the borehole, for among other things, removing drilling cuttings from the bottom of the borehole—may enter the formation and be lost from the drilling process. This loss of drilling fluid from the drilling process may cause a reduction in BHP and as a consequence cause a kick as the BHP falls below the formation pore pressure.
In order to overcome the problems of kicks and/or fracturing of formations during drilling, a process known as managed pressure drilling (“MPD”) has been developed. In MPD various techniques may be used to control the BHP during the drilling process. One such method comprises injecting gas into the drilling fluid/mud column in the drilling annulus (during the drilling process drilling fluid/mud is continuously circulated down the drillstring and back up through the annulus formed between the drillstring and the wall of the borehole being drilled and, as a result, during the drilling process a column of drilling fluid/mud is present in the annulus) to reduce the BHP produced by the column of the mud/drilling fluid in the drilling annulus. An MPD system using gas injection is illustrated in FIG. 1.
In MPD, the annulus may be closed using a pressure containment device. This device comprises sealing elements, which engage with the outside surface of the drillstring so that flow of fluid between the sealing elements and the drillstring is substantially prevented. The sealing elements may allow for rotation of the drillstring in the borehole so that the drill bit on the lower end of the drillstring may be rotated. A flow control device may be used to provide a flow path for the escape of drilling fluid from the annulus. After the flow control device, a pressure control manifold, with at least one adjustable choke, valve and/or the like, may be used to control the rate of flow of drilling fluid out of the annulus. When closed during drilling, the pressure containment device creates a backpressure in the borehole, and this back pressure can be controlled by using the adjustable choke or valve on the pressure control manifold to control the degree to which flow of drilling fluid out of the annulus/riser annulus is restricted.
During MPD an operator may monitor and compare the flow rate of drilling fluid into the drillstring with the flow rate of drilling fluid out of the annulus to detect if there has been a kick or if drilling fluid is being lost to the formation. A sudden increase in the volume or volume flow rate out of the annulus relative to the volume or volume flow rate into the drillstring may indicate that there has been a kick. By contrast, a sudden drop in the flow rate out of the annulus/relative to the flow rate into the drillstring may indicate that the drilling fluid has penetrated the formation and is being lost to the formation during the drilling process.
In some MPD procedures, gas injection may be used to control the BHP. In such MPD procedures, gas may be pumped into the annulus between the drillstring and the borehole wall (this annulus may be referred to as the “drilling annulus”) in order to reduce bottomhole-pressure while drilling. Often, the borehole is lined with a pipe that is referred to as a casing string that may be cemented to the borehole wall to, among other things, stabilize the borehole and allow for flow of drilling fluids, production of hydrocarbons from the borehole and/or the like. The drilling annulus may be formed by the annulus lying between the drillstring and the casing string.
Annular gas injection is an MPD process for reducing the BHP in a borehole. In many annular gas injection systems, in addition to lining the borehole with casing, a secondary annulus is created around the drilling annulus by placing an additional pipe around the casing for at least a section of the borehole. This secondary annulus may be connected by one or more orifices at one or more depths to the primary annulus, through which the drilling fluids flow. In this concentric casing type gas injection system, initiating the process of gas injection into the drilling annulus to reduce the BHP can be problematic as, among other things, injection of the gas into the borehole can produce large fluctuations in borehole pressure (the injected gas may create large, oscillating flow of the drilling fluid in the borehole) and achieving a steady-state in the borehole may take hours of unproductive time and/or require pumping large volumes of gas into the borehole. For example, if large gas injectors are used for gas injection, then large flows of drilling fluids may be produced between the gas injection pipe and the drilling annulus. Conversely, if small gas injectors are used, large pressures and gas volumes may be needed to force/inject the gas into the drilling annulus and these large pressures volumes may produce large oscillations in the pressure/flows in the drilling systems.